PURPOSE: To execute exactly an error recovery under a more practical environment, by recording the number of instruction steps and information related to a decision event.

CONSTITUTION: A machine instruction which has been executed, and a machine instruction which is re-executed in case an error has occurred are counted by a recovery counter 100, and also, a decision event in which some processing is an asynchronous interruption to be influenced on a decision of a final system state is recorded in an event recorder 123. In this state, if an error is generated in a computer system, the computer system is rest to the last check point system state, and an execution of the machine instruction is restarted, and each decision even is also repeated by an instruction point of the re-execution time of the same machine instruction. Accordingly, an error recovery can be executed exactly under a more practical environment.